In a transmission of this kind, the ratio is determined by the situation of the input-side and output-side tread radii of the encircling means on the cone pulleys, the same as the change thereof produced by an axial displacement of axially displaceable cone pulleys. The needed contact and adjusting forces of the cone pulleys are hydraulically applied by adequate setting devices. In the operation of the transmission, the hydraulic pressures are composed of one static and one dynamic portion; the dynamic portion being produced by centrifugal force action upon the hydraulic pressure medium enclosed in the setting device and rotating together with the cone pulleys.
In these transmissions, the pressure medium is usually supplied by way of an oil pump mechanically driven by a prime mover of the vehicle. Failure of the prime mover caused by damage then results in that no sufficiently high static hydraulic pressure exists. A purposeful actuation of a starting clutch or of the hydraulic setting devices is thus no longer possible for the transmission ratio. Particularly in the towing operation of such a vehicle, the problem arises that in the pressure chambers of said setting devices, the same as of the input-side starting clutch, the existing pressure medium is exposed to centrifugal forces which can lead to the adjustment of an undesired transmission ratio and/or to closing of the starting clutch.
Besides, said cone pulley transmission are usually designed so that upon the axially displaceable cone pulley of the transmission output or secondary shaft, one coil spring acts by which a minimum contact pressure is ensured on the encircling means. Due to the action of said coil spring, during pressureless shut off of the vehicle and of the transmission, there appears a LOW ratio or underdrive in direction of the lowest transmission ratio.
Mainly in case of cone pulley transmissions having a starting clutch on the side of the primary shaft, i.e., a starting clutch located between the prime mover and the input shaft of the transmission, during a towing operation, there occurs the effect that the cone pulley pairs are, as it were, driven by the vehicle driving wheels via the output shaft of the transmission. In said transmission ratio LOW, this results in very high rotational speeds and drag torques on the input or primary cone pulleys which can produce a skidding of the encircling means, the same as mechanical damages on the encircling means and on the cone pulleys.
To solve this mechanical problem, from DE 100 52 471 A1 is known a cone pulley CVT where a plate spring having a special spring characteristic line is operatively situated upon the axially displaceable primary cone pulley. This characteristic line is selected so as to have, in the area of the transmission ratio LOW, a minimum force and in direction to lower reduction ratios to rise up to a maximum spring tension. Shortly before the reduction ratio 1:1, the spring tension drops to a zero value and is in the so-called overdrive range absolutely inactive. It should be advantageous in this design that when the vehicle is shut off with a transmission reduction ratio 1:1, the spring characteristic line prevents its moving in direction to LOW in the absence of static hydraulic pressure.
It is deemed disadvantageous in this CVT that the reduction ratio, departing from the reduction ratio LOW at the start of a towing operation, is adjusted comparatively slow out of the damaging rotational speed range so that an abnormal closing of the primary-side starting clutch, the same as damages on the encircling means and on the cone pulleys, cannot be ruled out with certainty.
With this background, the problem on which the invention is based is to introduce a cone pulley CVT which exclusively, as a result of its mechanical design, ensures that during the towing operation no disadvantageously high rotational speeds and drag torques are generated on the primary side of the transmission.